Geothermal Heating and Cooling/Ground Loops and Open Loop Sources

The primary difference between Ground Source Heat Pumps and Air to Air heat pumps, is where we get/send the heat from/to. A Ground Source Heat Pump gets the heat from or sends the heat to the ground, whereas an air to air heat pump gets and sends the heat to the air. The equipment at the heat pump is slightly different because it uses a water to refrigerant coil, instead of a air to refrigerant coil, but once the primary coil is taken care of the heat pump systems are not all that different.

The greater efficiencies available with ground Source Heat pumps are therefore to do with the Ground Loop and how efficiently it allows the heat pump to work. There are two reasons why a Ground Source heat pump is more efficient, Ground Tempering, and Fluid Capacity for heat.

Ground Tempering, is caused because the ground does not act as a perfect conductor for heat, and thus, the temperature of the air above the ground goes through a wider range than the temperature of the ground itself.

Fluid Capacity for heat, has to do with how well a fluid/gas will carry heat. Air has much less capacity for storage of heat than water does, because of its gasseous state. The difference is quite large, and as a result systems that transfer heat with water can transfer more heat more quickly than systems that must rely on air as the transfer medium.

Since what we are interested in, in Geothermal heating and cooling is the efficiency of transfer of the heat, and therefore the increase in efficiency over other methods, using a fluid like water is a critical component to the raise in efficiency of the Geothermal system.

Once we have decided on a transfer medium, (Air or Water), the next thing, is how to transfer the heat from a source outside the building into the building. It might be possible to use an Air system to transfer heat from the ground, but that would require a large amount of air to be moved in order to do the transfer. A much smaller amount of water can be used and so the Ground Loop is designed to work with water.

Heat transfer from a location within the ground relies on three things:

How much surface area the transport medium has with the ground

How much heat conduction the ground has around the ground loop

What the temperature of the soil is at the time of transfer

The more surface area there is, the greater the heat transfer, the greater the conduction of the ground the more heat transfer, and the more radical the difference in temperature the more heat will be transferred.

Since the temperature of the ground stays relatively the same at each latitude, and lags the air temperature more the further towards the poles you go, we can say that the difference between the outside air and the ground temperature gets larger the more polar your latitude. Thus the ground loop itself is more efficient the more polar your latitude.

It should be noted that because water has more heat conduction capabilities than most types of ground, that swampy areas, and areas with high water tables are more efficient than dry sandy soil for ground loops.

For this reason many have been tempted to place their ground coils in rivers streams and lakes. The main objection to this practice is that removing heat from a fish habitat, will result in reducing the size of the habitat, and therefore the number of fish it can support. For instance it can be seen that putting a ground loop in a lake, will cause the ice depth to increase in that lake, and if the lake is small enough and enough ground coils are put into the lake, the effect will be to freeze the lake to the bottom killing off all the fish. For this reason the number of ground coils that are placed into a lake should be limited.

It might seem that since water is a better conductor of heat, that geothermal heat should be taken from a stream or well, and in some cases it has been done successfully with little ecological impact. However one of the problems with an open loop system, is that contaminants from the mechanical components of the system can penetrate an otherwise pristine environment, also if the installation is not done carefully enough, it can result in cross contamination between watersheds, etc. It is considered better form to keep the contaminants within the loop, by closing the loop. Besides closing the loop allows us to use anti-freeze and in sub-polar and polar climates, antifreeze is a must. The antifreeze used should be chosen to minimize ecological hazard in the case of a spill or crack in the water pipes.

This still doesn't mean that you can't use a stream or a well, in fact vertical ground loops are often achieved by digging a well, and putting a loop down it, sealing the water into its own strata by using parging to seal between strata. Parging for vertical loop systems is a legal requirement in some jurisdictions and so special techniques have been developed to make it easy to achieve.

In any case there are not enough rivers streams and lakes available for everyone to be able to use this technique, so it is an exception rather than the rule. Instead most ground loops are simply based on extraction of heat from the ground itself.